I think the prophecy was left deliberately vague, as most prophecies are. I'd always assumed the one who is already dead was Sheridan, although I've also heard people suggesting it could be Refa, as Londo poisoned him and was a potentially dead man walking. Refa might make more sense, as aside from the vision into the future in War Without End, I don't recall Londo with an opportunity to either kill or save Sheridan. Certainly if Londo didn't kill Refa, then Refa may have been the one to have Londo's fate instead.
In the Legions of Fire novels that Peter David wrote, Londo says he believes Morella said 'I' not 'eye', as in Londo must save himself but he doesn't see the consequences of his actions. However, in the B5 script books it is definitely written as 'eye', so I don't know about that. It could also be The Eye, the Centauri relic that the Shadows retrieved from the raiders in Signs and Portents.
But then again Londo had his own prophecy in the form of his death dream. So could he really have avoided his fate like Morella suggested?
Actually, thinking about it a bit more, Lady Morella tells him that Londo will be emperor, and he can't avoid that part of his fate. That's implying that everything between Point of No Return and the future scenes in War Without End cannot be changed. That would also explain why Sheridan is unable to change the future by going to Z'ha'Dum. I'm not sure I buy into that. If Londo hadn't poisoned Refa and then had him killed on Narn, Refa could have blocked Londo's ascension to Prime Minister and eventually Emperor. Refa would have been the Shadow's point of contact on Centauri Prime, so instead of them going directly to Cartagia, they would have negotiated with Refa to allow their ships to stay on Centauri Prime. Refa might then have had to take Londo's role in assassinating Cartagia to save the planet, Refa would then have positioned himself to become Emperor, and the Drakh may have gone after Refa in revenge instead of Londo. So by sparing Refa, Londo may have forced his fate onto Refa instead. On the other hand, if he hadn't killed Refa then Refa may have killed him anyway.